1. Field of the Invention
This invention relates to an electroluminescent (EL) display of an ac driven type.
2. Description of the Prior Art
EL displays of an ac driven type have an EL photoemissive layer, and a pair of electrodes formed on opposite surfaces of the EL photoemissive layer. When an ac voltage of a given amplitude or greater is applied between the electrodes, the EL photoemissive layer is excited to emit light so that display information can be indicated.
EL photoemissive layers are made of, for example, ZnS. The EL photoemissive layer made of ZnS exhibits the piezoelectric effect. Thus, in the EL display, when an ac voltage is applied between the electrodes, the EL photoemissive layer vibrates due to the piezoelectric effect so that noise tends to occur.
Various techniques have been developed to suppress or prevent such noise in an EL display. For example, Japanese published unexamined patent application 63-61285 and Japanese published examined patent application 63-30745 disclose that a spacer is provided between a display frame and an EL photoemissive layer, and the display frame and the EL photoemissive layer are sealed to prevent a leakage of noise from the EL photoemissive layer. EL displays disclosed by Japanese published unexamined patent application 63-61285 and Japanese published examined patent application 63-30745 tend to be large in size due to the presence of the spacer.
Japanese published unexamined patent application 63-249895 discloses that rising and falling edges in an applied ac voltage are made into gentle slopes to suppress or prevent noise generated by an EL photoemissive layer. The noise suppression technique disclosed by Japanese published unexamined patent application 63-249895 is ineffective to noise of a frequency equal to a frequency of the applied ac voltage.
Japanese published unexamined patent application 3-250580 discloses a method of driving EL elements. According to the method of Japanese application 3-250580, ac drive voltages having equal values and opposite polarities are applied to adjacent EL elements respectively to reduce electric-field induced noise.